Sunlight control glasses are commonly used in applications such as building glass windows and vehicle windows, typically offering high visible transmission and low emissivity. High visible transmission can allow more sunlight to pass through the glass windows, thus being desirable in many window applications. Low emissivity can block infrared (IR) radiation to reduce undesirable interior heating.
In low emissivity glasses, IR radiation is mostly reflected with minimum absorption and emission, thus reducing the heat transferred to and from the low emissivity surface. Low emissivity, or low-e, panels are often formed by depositing a reflective layer (e.g., silver) onto a substrate, such as glass. The overall quality of the reflective layer, such as with respect to texturing and crystallographic orientation, is important for achieving the desired performance, such as high visible light transmission and low emissivity (i.e., high heat reflection). In order to provide adhesion, as well as protection, several other layers are typically formed both under and over the reflective layer. The various layers typically include dielectric layers, such as silicon nitride, tin oxide, and zinc oxide, to provide a barrier between the stack and both the substrate and the environment, as well as to act as optical fillers and function as anti-reflective coating layers to improve the optical characteristics of the panel.
Low-emissivity coatings can also be engineered to provide desired shading properties. When sunlight reaches a window, a portion can pass through the window, a portion can be reflected, and a portion can be absorbed, (which can increase the temperature of various parts of the window). A portion of the absorbed heat can radiate to the inside of the building, thus increasing the temperature of the air in the building. Thus, when sunlight is incident upon a glass window, in addition to lighting the interior of the building, the incident solar radiation can also pass through the window to increase the temperature of the building. Solar Heat Gain Coefficient (SHGC) is defined as the fractional amount of the solar energy that strikes a window that contributes to warming the building. Other terms can also be used, such as solar shading property or Light to Solar Gain (LSG), which is used to describe the relationship between lighting and heating from solar irradiation. Light to Solar Gain is defined as the ratio of visible light transmission to solar heat gain coefficient. In the hot weather, it is desirable to have high LSG glass. For example, commercial glass coatings are generally recommended to have LSG greater than 1.8.
There can be a tradeoff between having high visible transmittance and high light to solar gain. Transparent glass can provide high light transmittance but also high solar gain, (e.g., low light to solar gain). Dark glass can provide low solar gain, but also low light transmittance. Low emissivity coatings incorporating silver can provide significant improvements in terms of both visible light transmittance and light to solar gain properties. However, further improvement in light to solar gain is difficult; for example, low emissivity coatings having thicker silver layers, or having multiple silver layers, (e.g., double silver layer or triple silver layer), can reduce the solar heat gain, but at the expense of lower light transmission.
Another desired characteristic of the low-emissivity glass coatings is a color neutral property, (e.g., colorless glass). The glass coatings should not exhibit observable hues, (e.g., more red or blue than is desired).
Another desired characteristic of the low-emissivity glass coatings is temperature stability, (e.g., similar performance and appearance before and after heat treatment). Since glass can be tempered, (e.g., heating the glass to 600-700° C.), the appearance of the low-emissivity coatings can change significantly during the heat treatment process. To accommodate the tempering changes, low-emissivity coatings can be provided in a temperable version (e.g., heat treated) and a non-temperable version (non-heat treated). This is inconvenient in a manufacturing environment since two separate types of inventory must be maintained. The film stack of the temperable version can be designed to have properties matching those of the non-temperable version.
It would be desirable to provide low-emissivity coatings that can provide high visible transmittance, high light to solar gain, color neutral, and thermal stability for color and optical performance.